Noise suppression system and method

ABSTRACT

A noise suppression assembly includes a first venturi configured to provide air to a combustion chamber and a second venturi configured to provide air to the combustion chamber. The noise suppression assembly also includes a valve subsystem configured to selectively limit air flow through at least one of the first venturi and the second venturi.

BACKGROUND OF THE INVENTION

Internal combustion engines have traditionally utilized a throttle to regulate airflow to the combustion chamber of a cylinder. Improvements in electronic engine control have enabled the intake and exhaust valves of the cylinder to control airflow, thereby eliminating the need for a traditional throttle. Such engines are commonly referred to as electronic valve actuated engines and/or throttleless engines. Throttleless engines can demonstrate improved performance, fuel economy, transient response, combustion stability, and emissions when compared to conventionally throttled engines.

However, the inventors herein have recognized a potential disadvantage of throttleless engines. Specifically, induction noise can be significantly greater when a throttle is not present. This is particularly true at low engine speeds, where traditional throttled designs have had a substantially closed throttle plate that reflects induction noise. Throttleless engines have a substantially unobstructed passage to the combustion chamber, which can significantly increase induction noise. Furthermore, the actuation of the electronic valves themselves may increase induction noise, thereby exacerbating induction noise issues.

SUMMARY OF THE INVENTION

A system and method for controlling induction noise are provided. In some embodiments, the system includes a noise suppression assembly including a first venturi configured to provide air to a combustion chamber and a second venturi configured to provide air to the combustion chamber. The noise suppression assembly also includes a valve subsystem configured to selectively limit air flow through at least one of the first venturi and the second venturi. In this manner, air flow can be controlled to satisfy the air requirements of an engine and simultaneously limit induction noise. During operation under low air requirements (such as low engine speed or low engine torque), one or more venturi may be at least partially blocked, thereby suppressing induction noise that could otherwise escape through the venturi.

In some embodiments, a noise suppression assembly may include one relatively large venturi and one relatively small venturi. During high engine air requirement operation, both venturis may supply air to the engine. During low engine air requirement operation, the large venturi can be closed, thereby suppressing induction noise that could otherwise escape through the large venturi.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a somewhat schematic view of a portion of an internal combustion engine including a noise suppression assembly.

FIG. 2 is a more detailed view of an exemplary noise suppression assembly including a dual venturi configuration.

FIG. 3 is a more detailed view of an exemplary venturi.

FIGS. 4-6 are cross-sectional views of the venturi of FIG. 3.

FIG. 7 shows an exemplary noise suppression assembly during low engine air requirement operation.

FIG. 8 shows the noise suppression assembly of FIG. 7 during high engine air requirement operation.

FIGS. 9-11 show experimental air-flow vs. manifold vacuum test results.

FIGS. 12-14 show experimental air-flow vs. overall sound pressure level test results.

FIG. 15 is a flow chart showing a method for controlling engine noise.

FIG. 16 is an example detailed view of an exemplary venturi.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENT(S) OF THE INVENTION

FIG. 1 shows an exemplary internal combustion engine 10, which can be used to power a vehicle such as a car or a truck. Engine 10 can include a plurality of cylinders, an exemplary one of which is shown at 12 of FIG. 1. Cylinder 12 includes a combustion chamber 14 that is partially defined by cylinder walls 16 and reciprocating piston 18. Cylinder 12 also includes intake valve 20, exhaust valve 22, fuel injector 24, and spark plug 26. Fuel injector 24 can be configured to spray fuel directly into combustion chamber 14 or the fuel injector can be aimed into an inlet air passage. In some embodiments, a cylinder may include two or more fuel injectors, which can be configured to inject fuel into different areas and/or two or more spark plugs, which can be used to facilitate complete combustion by initiating two flame fronts. Furthermore, it is within the scope of this disclosure to utilize two or more intake valves and/or exhaust valves.

Combustion chamber 14 is an area where chemical energy stored in fuel can be converted to mechanical energy. For example, an electronic engine controller 30 can be configured to control operation of intake valve 20, exhaust valve 22, fuel injector 24, and/or spark plug 26 to facilitate internal combustion within combustion chamber 14. In other words, intake valve 20 and exhaust valve 22 may cooperate with fuel injector 24 to create a desired air-to-fuel ratio in the combustion chamber. The controller receives various inputs from sensors, such as a measure of exhaust air-fuel ratio from sensor 110 (which can be a UEGO sensor or a HEGO sensor, for example), a measure of engine speed from speed sensor 112; a measure of airflow from mass air flow sensor 114, a measure of manifold pressure from pressure sensor 116, and various others not shown here, such as a measure of pedal position from a foot pedal position sensor, a measure of engine temperature from a coolant temperature sensor, a measure of airflow temperature from an air temperature sensor, and a measure of exhaust temperature from an exhaust temperature sensor. Further, these parameters can be estimated in controller 30 using estimation models and/or other techniques.

Spark plug 26 can be used to ignite the air and fuel mixture, thereby facilitating a controlled explosion that transfers chemical energy stored in the fuel into mechanical energy in the form of piston 18 recoiling from the explosion. Linear energy of piston 18 can be converted to rotational energy at a crankshaft 32, and such rotational energy can be utilized to drive one or more wheels of a vehicle.

As shown in FIG. 1, combustion chamber 14 is in fluid communication with an air supply 40 via an intake manifold 42. Intake valve 20 selectively allows air to flow into combustion chamber 14 from the intake manifold 42 according to instructions received from electronic engine controller 30. The intake manifold 42 provides an acoustic path through which noise associated with the operation of engine 10 can travel. Decreasing the escape of such noise is desirable.

Conventionally throttled engines utilize a throttle to regulate airflow through the intake manifold to the combustion chamber. In such engines, a partially closed throttle reflects most of the acoustic energy back to the engine while allowing adequate airflow to the cylinders. Without the throttle, there is virtually no resistance to the escape of acoustic energy. Therefore, throttleless engines can have substantially higher induction noise levels than conventionally throttled engines, especially when the engine is operated at low speeds.

The inventors herein have developed a system for decreasing induction noise, which, for example, can be used in throttleless engines. As schematically shown in FIG. 1, a noise suppression assembly 50 can be placed in fluid communication with intake manifold 42, so that air passing through the intake manifold passes through the noise suppression assembly. A noise suppression assembly can be located at the leading end of the intake manifold, as illustrated in FIG. 1. In other embodiments, the noise suppression assembly can be configured for inline placement as an intermediate segment of the intake manifold, or the noise suppression assembly can be configured for placement at the terminal end of the intake manifold, near a combustion chamber.

FIG. 2 schematically shows an exemplary embodiment of a noise suppression assembly 50 that includes a first venturi 60 and a second venturi 62 connected in parallel to an air box 64. Air box 64 is connected to intake manifold 42, which may include a zip tube or similar air passage that can be directed to one or more cylinders, such as cylinder 12. In some embodiments, air box 64 may include an air filter, and in some embodiments the air box may be eliminated. Air may pass through venturi 60 and/or venturi 62 to the cylinders, where it may be used to effectuate internal combustion. As described in detail below, venturi 60 and venturi 62 can substantially reduce induction noise while retaining adequate air flow for engine operation throughout a range of engine operating conditions.

FIG. 3 shows first venturi 60 apart from the rest of the noise suppression assembly. First venturi 60 is an air passage that includes an upstream portion 70, a throat portion 72, and a downstream portion 74. Upstream portion 70 includes a mouth 76 at the leading end of the venturi. As explained by the Bernoulli Equation, energy is conserved as fluid (air) moves through venturi 60. In particular, the sum of kinetic energy, pressure energy, and potential energy remains constant throughout the venturi. If the potential energy remains constant, an increase in fluid velocity results in a decrease in pressure, and vice versa. The decreased cross-sectional area of throat portion 72 causes the fluid velocity of air moving through the throat portion to increase relative to air moving through the upstream portion or the downstream portion. Therefore, air traveling through throat portion 72 will have a decreased internal pressure compared to air traveling through upstream portion 70 or downstream portion 74. However, the fluid velocity and internal pressure at upstream portion 70 can be made to be substantially equal to the fluid velocity and internal pressure at downstream portion 74 by sizing the upstream portion substantially the same as the downstream portion.

It should be understood that venturi 60 is provided as a nonlimiting example, and other venturi configurations are within the scope of this disclosure. For example, upstream portion 70 may be shortened and/or downstream portion 74 may be lengthened so that the downstream portion forms a greater percentage of the venturi. Conversely, downstream portion 74 may be shortened and/or upstream portion 70 may be lengthened so that the downstream portion forms a greater percentage of the venturi. In some embodiments, upstream portion 70 may effectively be reduced to a rim along the mouth of the venturi, and the throat portion of the venturi may be located substantially at the opening of the venturi.

FIG. 4 is a cross-sectional view of upstream portion 70, FIG. 5 is a cross-sectional view of throat portion 72, and FIG. 6 is a cross-sectional view of downstream portion 74. As illustrated in FIGS. 4-6, upstream portion 70 has a radius Ru, throat portion 72 has a radius Rt, and downstream portion 74 has a radius Rd. In the illustrated embodiment, venturi 60 is shaped with a circular cross-sectional geometry, however, it is within the scope of this disclosure to configure a venturi with a noncircular cross-sectional geometry. The cross-sectional area of a circle is πr², where r is the radius of the circle. Therefore, the cross-sectional area of throat portion 72 is A_(t)=πR_(t) ². Similarly, the cross-sectional area of upstream portion 70 is A_(u)=πR_(u) ²; and the cross-section area of downstream portion 74 is A_(d)=πR_(d) ².

As illustrated in FIGS. 3-6, A_(t) is less than A_(u), and A_(t) is less than A_(d). Although not required, A_(u) equals A_(d) in the illustrated embodiment. The relatively small cross-sectional area of throat portion 72 can at least partially reflect, absorb, block, and/or otherwise suppress passage of acoustic energy through the venturi. It has been found that the amount of acoustic energy that is suppressed is related to the size of the throat portion, with smaller throat portions suppressing more acoustic energy than larger throat portions. Smaller throat portions may also contribute to larger manifold vacuum values. Therefore, throat size may be selected to achieve a desired balance of acoustic energy suppression, which can be measured as an overall sound pressure level (OSPL), and a desired manifold vacuum level (MAV). In other words, a reduction ratio A_(u)/A_(t) (and/or A_(d)/A_(t)) may be selected to achieve a desired reduction in induction noise while retaining sufficient air flow and/or engine efficiency. It is within the scope of this disclosure to use a venturi with virtually any reduction ratio (A_(u)/A_(t) and/or A_(d)/A_(t)), including, but not limited to, reduction ratios of 2 (50% throat area), 4 (25% throat area), 6⅔ (15% throat area), and 10 (10% throat area). FIGS. 3-6 show a venturi having a reduction ratio of 4, that is the cross-sectional area of throat portion 72 is 25% the cross-sectional area of upstream portion 70 (and downstream portion 74) and R_(t) is half of R_(u) (and R_(d)).

Venturi 62 can be configured with the same general characteristics as venturi 60, namely a relatively narrow throat portion between relatively wider upstream and downstream portions. The reduction ratio of venturi 62 can be the same as the reduction ratio of venturi 60, or venturi 60 and venturi 62 can be configured with different reduction ratios. Furthermore, venturi 60 and venturi 62 can be similarly sized, or venturi 60 and venturi 62 can be differently sized, as is shown in FIG. 2. By differently sized it is meant that, compared to the other venturi, one venturi has a larger volume, air throughput capacity, throat portion radius (R_(t)), cross-sectional area at the throat portion (A_(t)), upstream portion radius (R_(u)), cross-sectional area at the upstream portion (A_(u)), downstream portion radius (R_(d)), and/or cross-sectional area at the downstream portion (A_(d)). In the illustrated embodiment, venturi 60 is larger than venturi 62. In particular, venturi 60 has a larger R_(u), R_(t), and R_(d). However, it should be understood that a venturi having similar R_(u) and R_(d) measurements, but a smaller R_(t) measurement, could be used. Compared to venturi 62, venturi 60 can channel relatively more air in a given period of time. However, because of its relatively small size, venturi 62 can suppress more acoustic energy than venturi 60.

An engine can operate throughout a plurality of engine operating conditions, characterized by engine speed (RPM), load, torque, etc. Increasing demands on an engine generally correspond to increased air requirements. As is schematically shown in FIG. 2, noise suppression assembly 50 can include a valve subsystem 80 for selectively opening and closing venturi 60 and/or venturi 62 to allow air to respectively pass therethrough, and thereby fulfill the air requirements of an engine. In some embodiments, the valve subsystem may include a plurality of valves that are individually configured to selectively open and close a single venturi, while some embodiments may have one or more valves configured to jointly control two or more venturis. In some embodiments, the valve subsystem may include one or more valves that are configured to restrict air flow without completely closing a venturi. In some embodiments, the valve subsystem may include a single valve, which is configured to selectively restrict air flow through only one venturi. For example, as shown in FIGS. 7 and 8, the valve subsystem can include a valve 82 that is configured to selectively restrict air flow through venturi 60 based on instructions received from electronic engine controller 30. Closing venturi 60 suppresses the escape of acoustic energy through venturi 60. Valve 82 can be configured as a gate valve, knife-gate valve, butterfly valve, or other suitable valve for selectively restricting air flow.

FIGS. 7 and 8 respectively illustrate example operation of valve subsystem 80 during low engine air requirement operation and high engine air requirement operation. During low engine air requirement operation, valve subsystem 80 can close venturi 60, thereby decreasing net air flow and increasing induction noise suppression. Conversely, during high engine air requirement operation, valve subsystem 80 can open venturi 60, thereby increasing net air flow to the engine. The engine controller can be configured to change the configuration of valve subsystem 80 before, after, or at the same time as when an engine changes from low engine air requirement operation to high engine air requirement operation. It should be understood that the above description is to a noise suppression assembly with two operating modes corresponding to two air requirement operation states. It is within the scope of this disclosure to use a noise suppression assembly configured for three or more air requirement operation states. Such noise suppression assemblies can include three or more venturis and/or a valve subsystem that is configured to close one or more of the included venturis in incremental steps, so that such a venturi can accommodate a limited percentage of its maximum throughput.

An air requirement operation state may be determined by engine RPMs, load, torque, oxygen levels, and/or other parameters corresponding to the amount of air that can be utilized for internal combustion. For example, an air requirement operation state can correspond to the revolutions per minute of the engine and can be monitored by electronic engine controller 30. As a nonlimiting example, the electronic engine controller can be configured to treat anything less than 1500 RPMs as low engine air requirement operation, and anything equal to, or greater than, 1500 RPMs as high engine air requirement operation. Alternatively, the air requirement can be determined by measuring airflow through the engine, such as via mass air flow sensor 114 and/or manifold pressure sensor 116.

As shown in FIG. 7, during low engine air requirement operation, valve 82 may effectively close venturi 60, thereby limiting air flow to venturi 62. Closing venturi 60 can advantageously suppress induction noise while allowing for sufficient air flow through venturi 62. As shown in FIG. 8, during high engine air requirement operation, valve 62 can be opened so that air may flow through venturi 60 as well as venturi 62, thereby increasing net airflow. Although induction noise may be greater than the configuration and operating condition described with reference to FIG. 7, the shape of venturi 60 and venturi 62 helps suppress induction noise while retaining sufficient air flow. In particular, a venturi having a throat radius R_(t), upstream radius R_(u), and downstream radius R_(d) can facilitate greater air flow than a passage with a constant cross-sectional radius equal to R_(t), and can suppress more induction noise than a passage with a constant cross-sectional radius equal to R_(u) or R_(d).

The pressure drop in the throat portion of a venturi, such as venturi 60 and/or venturi 62, can be used as an introduction location for stored evaporative emissions. A conventional throttleless engine may have essentially atmospheric pressure in the intake manifold, thereby hindering flow between a canister and the manifold because no pressure drop is present. However, a venturi can be used as a vacuum source to drive flow from the canister to the intake manifold. Further, unlike conventional engines, as engine speed and load increase, the pressure drop in the venturi increases. This allows purge flow throughout the operating region of the engine. The venturi can also be used to provide a vacuum source for brake boost and/or exhaust gas recirculation.

Noise suppression has been tested using differently sized venturis. In particular, three venturis were designed such that the effective open area at the throat was 50%, 25%, and 15% of the original area at the mouth of the upstream portion of the venturi. Each of these cases represents a closed valve situation corresponding to low engine air requirement operation. Engine speeds of 650 rpm, 2500 rpm, and 5500 rpm were used as test cases. FIGS. 9-11 show the resulting manifold vacuum vs. flow rate for the different engine speed and venturi size combinations. As can be seen, manifold vacuum can increase with increasing flow rate. Flow losses are associated with the induction system, including the loss across the various venturis. Higher flow rates generally lead to higher flow losses. Note the minimal vacuum production (<1 kPa) in the 650 rpm case at the low loads, even for the smallest venturi.

FIGS. 12-14 show experimental induction noise levels (measured as overall sound pressure levels). For the 650 rpm case there is substantial OSPL reduction (˜20 dB) for a venturi having a throat area 15% as large as an upstream area. As stated earlier, this is accomplished with minimal vacuum production (see FIG. 9) of less than 1 kPa near idle loads. At higher engine speeds, the smaller inlet venturis, especially the 25% and 15% areas, are relatively effective at reducing induction noise. However, this may be at the cost of increased fuel consumption because the resulting manifold vacuum values are relatively large.

One method for controlling throttleless engine noise is now described. The method includes first placing a first venturi intermediate a combustion chamber and an air supply, wherein the first venturi includes a first throat portion having a smaller cross-sectional area than adjacent upstream and downstream portions of the first venturi. As described above with reference to FIGS. 3-6, such a venturi can be variously sized to achieve a desired balance of noise suppression, air flow, and fuel efficiency. The method also includes placing a second venturi intermediate the combustion chamber and the air supply in parallel with the first venturi, wherein the second venturi includes a second throat portion having a smaller cross-sectional area than adjacent upstream and downstream portions of the second venturi. Although not required, the second venturi can be sized smaller than the first venturi. The method also includes selectively blocking air flow through at least one of the first venturi and the second venturi. For example, a gate valve may be used to block air flow through a relatively large first venturi during low engine air requirement operation.

Referring now to FIG. 15, a routine for controlling the gate valve in the venturi system is described. First, in step 1510, engine operating conditions are determined, such as engine speed, load, airflow, and/or temperature. Then, in step 1512, a determination is made as to whether the gate valve should be activated (for example, operated to close or partially close) based on the determined engine operating conditions. For example, in one embodiment, the valve is activated when engine airflow is below a predetermined threshold value. In another example, the valve is activated when engine speed and load are within a given set of limit values. In still another example, the valve is activated based on engine temperature, or during engine starting/stopping conditions.

When the answer to step 1512 is Yes, the routine continues to step 1514 where a desired gate valve position is determined based on the determined operating conditions. For example, the valve can be simply set to a fully closed/open position, or to an intermediate position in some examples during selected conditions. Then, in step 1516, the routine adjusts the commanded signal sent to the valve to place it in the desired position.

In this way, improved noise suppression can be obtained.

Referring now to FIG. 16, an alternative embodiment is described where the smaller venturi is used as a vacuum source, and/or a location for introducing fuel vapors and/or exhaust gas recirculation (EGR). Although in another example, the larger venturi can be used for such purposes, or both can be used. Specifically, the venturi creates a localized zone of increased vacuum relative to the remaining portion of the intake system. Since vacuum can be advantageously used for powering actuators (such as vacuum actuated valves and/or a brake booster coupled to the brake system of the vehicle), and as a draw for fuel vapors stored in an evaporative emission system, or as a draw for exhaust gas recirculation systems. As shown in FIG. 16, a line 1610 can be added at or near the throat of the venturi to tap the vacuum as a source, and/or introduce purge vapors or EGR. In an alternative embodiment, the line can be introduced at the throat of the larger venturi, or each venturi can have a line. In such a case, one can be used as a vacuum source, and the other used for fuel vapors and/or EGR. In still another embodiment, the line can be place slightly before or after the throat to adjust the vacuum potential, if desired.

Further, since the venturi can reduce the engine vibration and acoustic noise, it may also be able to reduce engine pulsations that can corrupt a mass air flow sensor measurement. As such, placing a mass air flow sensor (such as sensor 114) near a throat of one or both of the venturi may provide beneficial measurement of flow due to reduce engine pressure pulsations. However, robustness and durability of the sensor typically require clean air, and so the mass airflow sensor needs to be placed after some air filtering media. Thus, if used in the venturi, some air filtering could be added upstream of the venturi.

Although described above in the context of a four stroke internal combustion gas engine, it should be understood that it is within the scope of the present disclosure to utilize a plural venturi system to decrease perceived noise in virtually any application in which controllable air supply is desired. While the present disclosure has been provided with reference to the foregoing operational principles and embodiments, it will be apparent to those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope defined in the appended claims. For example, an the above features can be used with throttled engines having an electronically controlled throttle, or mechanically actuated throttle, if desired. The present disclosure is intended to embrace all such alternatives, modifications and variances. Where the disclosure or claims recite “a,” “a first,” or “another” element, or the equivalent thereof, they should be interpreted to include one or more such elements, neither requiring nor excluding two or more such elements. 

1. A noise suppression assembly, comprising: a first venturi configured to provide air to a combustion chamber; a second venturi configured to provide air to the combustion chamber; and a valve subsystem configured to selectively limit air flow through at least one of the first venturi and the second venturi.
 2. The noise suppression assembly of claim 1, wherein the first venturi is sized for greater maximum throughput than the second venturi.
 3. The noise suppression assembly of claim 2, wherein the valve subsystem includes a valve configured to selectively close the first venturi.
 4. The noise suppression assembly of claim 1, wherein the valve subsystem is configured to close the first venturi during low engine air requirement operation.
 5. The noise suppression assembly of claim 1, wherein the valve subsystem is configured to open the first venturi during high engine air requirement operation.
 6. The noise suppression assembly of claim 1, wherein the first venturi is positioned in a parallel airflow configuration with the second venturi.
 7. The noise suppression assembly of claim 1, further comprising an air box intermediate the combustion chamber and the first and second venturi.
 8. The noise suppression assembly of claim 1, wherein the second venturi includes a mouth and a throat, and wherein a cross-sectional area of the throat is at most half a cross-sectional area of the mouth.
 9. The noise suppression assembly of claim 1 further comprising a filter located upstream of said first venturi.
 10. The noise suppression assembly of claim 9 wherein said filter is located upstream of said second venturi.
 11. The noise suppression assembly of claim 1 further comprising a mass air flow sensor.
 12. A noise suppression assembly, comprising: a first passage configured to be positioned intermediate a combustion chamber and an air supply, wherein the first passage includes a first upstream portion, a first downstream portion, and a first throat portion between the first upstream portion and the first downstream portion, wherein a cross-sectional area of the first throat portion is less than a cross-sectional area of the first upstream portion and less than a cross-sectional area of the first downstream portion; a second passage configured to be positioned intermediate the combustion chamber and the air supply in parallel with the first passage, wherein the second passage includes a second upstream portion, a second downstream portion, and a second throat portion between the second upstream portion and the second downstream portion, wherein a cross-sectional area of the second throat portion is less than a cross-sectional area of the second upstream portion and less than a cross-sectional area of the second downstream portion; and a valve configured to selectively restrict air flow from the air supply to the combustion chamber through the first passage.
 13. The noise suppression assembly of claim 12, wherein the first throat portion has a greater cross-sectional area than the second throat portion.
 14. The noise suppression assembly of claim 13, wherein a minimum cross-sectional area of the second throat portion is at most 50% a minimum cross-sectional area of the first throat portion.
 15. The noise suppression assembly of claim 13, wherein a minimum cross-sectional area of the second throat portion is at most 25% a minimum cross-sectional area of the first throat portion.
 16. The noise suppression assembly of claim 13, wherein a minimum cross-sectional area of the second throat portion is at most 15% a minimum cross-sectional area of the first throat portion.
 17. The noise suppression assembly of claim 12, wherein the first passage has a greater maximum throughput than the second passage.
 18. The noise suppression assembly of claim 17, wherein the valve is configured to block air flow through the first passage during low engine air requirement operation.
 19. A method of controlling an engine, where the engine includes a first venturi intermediate a combustion chamber and an air supply, wherein the first venturi includes a first throat portion having a smaller cross-sectional area than adjacent upstream and downstream portions of the first venturi, and a second venturi intermediate the combustion chamber and the air supply in parallel with the first venturi, wherein the second venturi includes a second throat portion having a smaller cross-sectional area than adjacent upstream and downstream portions of the second venturi, the engine further including a valve coupled to at least one of the first and second venturi the method comprising: adjusting the valve to adjust an amount of flow through the valve as engine operating conditions vary.
 20. The method of claim 19, wherein said adjusting selectively blocks air flow based on engine air requirements.
 21. The method of claim 20 wherein said selective blocking includes blocking the first venturi during low engine air requirement operation.
 22. The method of claim 19, wherein the first venturi has a greater maximum air throughput than the second venturi.
 23. The method of claim 19, wherein a cross-sectional area of the first throat portion is less than a cross-sectional area of the second throat portion.
 24. The method of claim 23, wherein said adjusting includes closing the first venturi during low engine air requirement operation. 